Many types of fuel cells are known in the art, such as solid oxide fuel cells, molten carbonate fuel cells, phosphoric acid fuel cells and proton exchange membrane (PEM) fuel cells. Conceptually, the operation of a fuel cell is very simple. An electrolytic medium separates an anode and a cathode, between which electricity is produced when a fuel is introduced to the anode, an oxidizer is introduced to the cathode, and the cell is maintained at the proper temperature. The electrolytic medium allows an ionic species to travel between the cathode and the anode. The reaction products are relatively simple and benign, typically including carbon dioxide and water, thus minimizing environmental concerns. In contrast to other energy sources, such as internal combustion engines, fuel cells are simpler, less noisy, do not pollute, demonstrate high efficiencies, and create electricity directly.
In practice, however, a fuel cell power supply can be relatively complex, as considerable hardware can be required to support the fuel cells, which are typically arranged in an cell stack assembly (CSA). Such hardware can include a thermal management subsystem for maintaining the CSA at the proper operating temperature, a fuel processing subsystem that can include fuel reformers and shift converters for generating a hydrogen fuel from a hydrocarbon fuel, and a water management subsystem for recovering water generated by the operation of the fuel cell(s) to reduce the need for external water. Desulfurization of the fuel is often also required. The various subsystems are often interrelated, for example including heat exchangers, blowdown coolers or condensers for transferring heat and/or water from one subsystem to another.
Water management is particularly important in a Proton Exchange Membrane (PEM) fuel cell power supply. Water generated at the cathode should be removed to avoid flooding the cathode and preventing the oxidant from reacting at the cathode. Furthermore, water is dragged through the membrane by H.sup.+ protons to the cathode, drying out the anode and adding to the water that must be removed from the cathode. Such drying of the anode, or even drying of the cathode due to improper water management, can damage the proton exchange membrane. Accordingly, water is typically added to the fuel input of the anode, and removed from the cathode in controlled manner by exhausting of the cathode external to the system. Other subsystems, such as a fuel reformer, can require water. It is desirable that water use and generation be balanced such that the power supply be self-sufficient and does not require water from an external source. Water balance concerns thus add complexity to PEM fuel cell power supplies.
To date, fuel cells power supplies, such as those based on PEM cells, have not found widespread use, such that their environmental and other benefits can be fully realized, in part because of the complexity and associated cost of existing fuel cell power supplies. Such a situation is far from satisfactory, as environmental and other concerns with the drawbacks of traditional power sources (such as internal combustion engines and coal or oil fired electrical power generation plants) are unlikely to become less pressing.
Accordingly it is an object of the invention to reduce the complexity and/or the cost of fuel cell power supplies.
It is another object of the invention to provide methods and apparatus for improving water management in a PEM fuel cell power supply.
Other objects of the invention will in part be apparent and in part appear hereinafter.